1. Field of the Invention
The present invention relates to a blow-molded vessel having a multi-ply structure and a method for the production thereof. More particularly, the blow-molded vessel of the present invention as a multi-ply structure composed of laminated different kinds of resins and has improved appearance and physical properties.
2. Prior Art
Recently, various hollow vessels comprising thermoplastic resins have widely been used in the fields of cosmetics, foods and drugs because of their advantages such as lightness, safety against breakage and the like. Particularly, with progress of blow molding technique, especially biaxial orientation blow molding technique, development of hollow vessels which comprise a polyethylene terephthalate having crystallizability and excellent mechanical properties has rapidly been promoted. For example, U.S. Pat. No. 3,733,309 discloses this kind of hollow vessel.
However, the vessels produced from thermoplastic resins, mainly a polyethylene terephthalate, by biaxial orientation technique do not necessarily satisfy all performances. Particularly, these vessels are not suitable for packing foods which require good gas barrier properties because they have poor gas barrier properties against oxygen.
Besides, depending upon the contents to be packed in the vessels, it is required to color the vessels and further to screen the contents from ultraviolet light. Usually, in order to provide these performances to the vessels, additives such as dyes or pigments, ultraviolet absorbing agents and the like are formulated in the thermoplastic resins to be used before molding. However, there is such a disadvantage that the additives are extracted from the molded vessels by the contents packed therein, which causes a hygienic problem and spoils the flavor of the contents. It has been known that a multi-ply structure composed of different kinds of resins having different properties, an innermost layer of which is free from the additives, can be employed to eliminate the above disadvantage. For example, U.S. Pat. No. 3,882,259 discloses a multi-ply vessel which comprises an inner and an outer layer each composed of a polyethylene and a middle layer composed of a blend of an ionomer and an ethylene/vinyl alcohol copolymer. This vessel is produced by extruding the three layers simultaneously to form a three-layer tube, pinching off the tube while it is hot to seal one end thereof to form the bottom part of the vessel and then blow-molding the pinched tube (parison) in a desired shape of the vessel. However, according to this process, the molded vessel has such a structure that the middle layer thereof is exposed to the atmosphere at the open end (mouth part) of the vessel, which causes some disadvantages such as deterioration of gas barrier properties due to the effect of water, and the tendency toward loss of at the non-orientated part of the vessel due to crystallization and the like. Further, the present inventors have previously found that a three-layer vessel which comprises an inner and an outer layer each composed of a polyethylene terephthalate and a middle layer composed of a metaxylylene group-containing polyamide resin has excellent gas barrier properties and have filed patent applications directed to this vessel (Japanese Patent Laid Open Publication Nos. 64839/1981 and 64866/1981 and International Application WP 81 01265). Only one process for the production of the vessel which is disclosed in these patent applications is a successive laminate modling process (i.e. stepwise formation of the inner, middle and then outer layers). However, when a multi-ply parison is formed by using a crystalline thermoplastic resin according to this process, occasionally, it is difficult to obtain a transparent multi-ply parison because an opaque portion appears around the gate mark of the bottom part or haze develops on the entire parison depending upon the variation of cooling efficiency at molding and the shape of the parison. As a result of the present inventors' further study, it has been newly found that this disadvantage results from the following:
In case of a successive injection laminate molding, a molten crystalline thermoplastic resin which forms the inner layer, particularly, a polyethylene terephthalate, a polyamide or the like is solidified in a transparent amorphous state by quenching. However, when the middle or the outer layer is formed sequently as a molten resin, it is difficult to quench because the molten resin flows on the surface of the inner amorphous resin layer or the middle layer (in case of formation of the outer layer) and is cooled through the inner layer or both inner and middle layers, and hence, the interfacial part of the layers becomes opaque due to heat crystallization of the resins of both layers.
The crystallization as above causes not only deterioration of blow moldability of the resulting multi-ply parison but also other disadvantages such as lowering of the interfacial adhesion between both resins, deterioration of physical properties of the hollow vessel to be obtained and the like. Furthermore, in the above patent applications, the vessel also has such a structure that the middle layer thereof is exposed to the atmosphere at the mouth part of the vessel, which causes loss of transparency at the non-orientated part of the vessel due to the effect of water.